Marshall Valvestate blowing fuses.

Hey, i've got a marshall valvestate 8100 head lying around that i'm trying to fix up. It's been blowing fuses and i'm having trouble identifying what's causing this. I've been systematically pulling and replacing components and this is what i got:

Took board out and placed on insulating material = blow (so no shorts with the case)

So i'm guessing the problem's on the other side of the rectifier. I can't see any shorts on the board or any noticeably burntout components (though some of the solderpads look burnt, but that may be due to dodgy soldering at the factory) I can't see it being a resistor or anything small. I'm considering removing the main transistors and the heatsink and see what happens then. Any ideas?